Galvanized malleable iron and method of producing same



Patented Jan. 23, 1934 PATENT OFFICE GALVANIZED MA-LLE ABLE IRON AND METHOD OF PRODUCING SAME Votaw S. Durbin, St. Louis, M0.

N Drawing. Application September 11, 1931 Serial No. 562,424

4 Claims. (Cl. 9170.2)

The general object of this invention is to enable malleable iron castings to be galvanized without the liability of hardening the metal or to v any extent decreasing or impairing its malleable, 5 ductile, or tensile characteristics.

Normal malleable iron when broken shows a dark gray fracture. If it is not correctly annealed, or annealed at the proper temperature, it will show a white, sparkling fracture. When the metal shows a white fracture, it is practically useless for any of the purposes for which malleable iron is ordinarily employed, dueto the fact that it is brittle and of very low ductility.

In the galvanizing of malleable iron castings by conventional methods, it is a common experience to find that a relatively high percentage of the castings will be hardened by the galvanizing process, and, so far as I am aware, no method of galvanizing malleable iron castings withoutat the same time incurring the danger of hardening the metal has been devised prior to this present invention.

In the galvanizing of malleable iron as generally practiced, the zinc for galvanizing is first heated to a temperature of approximately 900 F. The malleable iron casting, having been first thoroughly cleaned, either by subjecting it to an acid bath, or to a sand blast, or in any other preferred manner, is then immersed in the molten zinc and left therein for a sufficient length of time for the casting to be brought to the temperature of the zinc. This time will vary according to the size and thickness of the casting. The casting is then removed from the zinc bath, dipped in water of a temperature of about 72 F. and permitted to remain therein untilit cools oil, which may require from one to five minutes.

In a large percentage of cases, the malleable iron will be hardened by this process and ren- 40 dered useless. That is to say, it is no longer, malleable, but brittle, and posseses little or no ductility. Such result it is the object of my invention, to prevent.

According to thisinvention, after removal from the cooling bathof water, the galvanized casting is then placed in an annealing oven and brought to a temperature of approximately 1200" F. Theoretically, this temperature would range below 1200? F. down to-about 1000 F. and above 1200" F. up to approximately 1360 F. Below a temperature of 975 F. down to approximately 875 R, which is what is known in the art as the blue heat range, I have found that there is great danger of the malleable iron becoming 55 hardened. On the other hand, if the temperature is raised above 1360" F., the free carbon will combine with the iron, or ferrite, and when suddenly cooled the metal is very liable tobe made hard and brittle.

' It is my view that'the hardening of malleable iron at temperatures within the blue heat range is caused by the unequal expansion of'the elements comprising the alloy. That is to say, some of the elements will undergo a maximum degree of expansion within the blue heat range, while others will only be relatively slightly expanded. This unequal expansion destroys the structural characteristics of malleable iron, and results in making it relatively. hard, brittle and non-ductile. At the higher temperatures, that is, at temperatures beyond the blue heat range and below the critical combining temperature, or 1360 F., all

of the elements will be expanded approximately to the maximum degree with the result that there is no relative change in the structural characteristics of the metal. g

There are various theories to account for the change in malleable iron, under certain temperature conditions, from one having a black or dark gray fracture to one having a white fracture, but I consider it unnecessary to deal with these theories, or to attempt to reconcile them with my own. It is sufilcient for the purpose of this invention to state that the fact that malleable iron is rendered brittle when subjected to a'temperature approximating that of the blue heat range is well known in this art. Thus, in an article published by the American Foundrymens Association entitled .Practical aspects of whitefraoturemalleable it is stated:

But the surest way to develop a white fracture in metal having a tendency in that direction, is to heat the casting, after annealing, up to 850 degrees and then quench in air or water. This is what happens in the hot dip galvanizing process and gives rise to galvanizing embrittlement. Castings which after annealing do not show white fracture even atlow temperature may become white fracture castings after galvanizing."

Notwithstanding this knowledge, the hot dip galvanizing process is commonly practiced, with theresult, as stated, that many castings are ruined so far as their malleable feature is concerned.

This result is entirely eliminated by reheating l the galvanized metal after cooling, as above explained. Such reheating, even in cases of white fracture metal, apparently restores the crystalline or grain structure characteristic of malleable iron, as I have found by actual experience that galvanized castings showing a white fracture, when heated to a temperature of 1200 F. will,

- after removal from the annealing oven and cool- Not only does my invention enable malleable iron castings to be galvanized without danger of embrittlement, but the character of the gelvanizing is found to be far superior to that resulting from the employment of the conventional method. I will briefly describe the result of galvanizing according to my method.

At a temperature of approximately 1200 F., the zinccoating on the metal is, of course, rendered molten, and as aresult it permeates the surface of the casting to a depth ranging from about .001 to .007 of an inch. Upon bending a piece of metal galvanized according to my improved method, there is shown an exterior layer, which will break or scale oil, which corresponds to the coating produced by galvanizing in the ordinary way; and below this there is a galvanized surface, which is indistinguishable, except as to color, from the surface of the casting. That is to say, such inner coating is homogeneous with the metal at the surface of the casting.

Actual salt spray tests, such as commonly employed, conducted on metals galvanized by the ordinary hot dip process and by my process, respectively, have demonstrated that the surface of the latter'possesses a much greater resistance to oxidation than the former. In fact, metal galvanized according to my process isrendered practically rust-proof.

Thus, after subjecting the two specimens of galvanized metal to a salt spray for a period of sixty hours the metal galvanized by the hot dip process showed several spots of oxidized iron, whereas my metal showed none whatever. This test, it is recognized, demonstrated only relative superiority of a casting galvanized according to my process, as compared with one galvanized according to the conventional method. The test was not continued beyond the point of establishing this fact. In other words, it was sought to prove that malleable iron could be ga vanized according to my method without embrittlement of the metal and that the galvanizing would be in no way inferior to that resulting from practicing the usual hot dip method.

The superiority of the galvanizing resulting from practicing my method is due to the fact that a portion of the zinc enters the pores of the iron at the surface of the casing, and to this is added the usual outer coating characteristic of the ordinary galvanizing process. It will be apparent to those skilled in the art that metal thus treated will have a much higher resistance to oxidation than one having only a coating applied to its surface.

After the metal is reheated according to my invention, as explained above, it is taken from the oven and either allowed to cool in the air or it may be cooled in a water or oil bath.

Repeated tests have shown in all cases that the metal thus galvanized and reheated possesses as great ductility as the original metal before galvanizing, and in many cases an increase in ductility has been shown. The tensile strength of the metal is also in all cases either as great as, or greater than, that of the original metal before galvanizing.

I claim:

1. The method of galvanizing, without causing embrittlement, castings having an iron base and previously annealed to convert them into malleable iron, which consists in dipping the castings into molten zinc, cooling the same and then reheating the castings to a temperature of approximately 1200 F.

2. The method of galvanizing, without causing embrittlement, castings having an iron base and previously annealed to convert them into malleable iron, which consists in dipping the castings into molten zinc maintained at a temperature of approximately 900 F., cooling the same, and then reheating the castings to a temperature of approximately 1200 F.

3. The method of galvanizing, without causing embrittlement, castings having an iron base and previously annealed to convert them into malleable iron, which consistsin dipping the castings into molten zinc, allowing the coated castings to cool, reheating the coated castings to a temperature of approximately 1200 F., and then cooling the castings.

4. The method of galvanizing, withoutcausing embrittlement, castings having in iron base and previously annealed to convert them into malleable iron, which consists in dipping the castings into molten zinc maintained at a temperature of approximately 900 F., allowing the castings to cool, reheating the castings to a temperature of approximately 1200 F., and then cooling the castings.

VOTAW S. DURBIN. 

